Cellular communication networks such as the GSM network system have widely spread in recent years with the increase of the demand for mobile communication.
FIG. 1 shows a rough outline of a part of a cellular communication network. Generally, the network area served by the network is composed of individual cells C1, C2, C3 . . . , and/or c1, . . . c7. Each cell in turn is served by a respective base station BS or base transceiver station BTS (not shown in every cell). The coverage area of such a base station BS is defined by the cell radius R and/or r. The coverage area and cell radius are adjustable by the transmit power used by the transmitter of the base station BTS.
Thus, dependent on the transmit power of the BTS used, the network may be composed of so-called macrocells meaning a cell covering a large area (with a cell radius R of for example up to 30 km, or even more. For example, GSM standard allows cells of radius 35 km, while with special well-known cell extension techniques, the radius may be extended—in areas with prevailing radio propagation condition which allow this—up to 120 km). Examples of such macrocells are illustrated in bold (solid and dashed) lines in FIG. 1 and labeled C1, C2, C3, respectively. On the other hand, a low set transmit power leads to a cellular network composed of microcells meaning a cell covering a small area only, e.g. cells c1 to c7 in FIG. 1. (Note that in microcell network layouts the number of base stations per area—compared to macrocell layouts—needs to be increased so that there do not arise gaps between the coverage areas of the microcells.) Although typically microcells have a cell radius r not exceeding 500 m, this is not limiting for the present invention. Rather, a microcell in the present specification is to be understood as a cell covering a small area such that the coverage area of plural small cells (c1, . . . , c4) is comprised in the coverage area of a large cell (C1), as it is illustrated by way of example in FIG. 1. Also, as illustrated in FIG. 1, cellular networks may adopt a cellular structure in which a macrocell layout is overlaid to a microcell layout. Nevertheless, a microcell layout may be provided for without an overlaid macrocell layout (and vice versa). Microcell layouts are preferably used in “hot spots” of the network where a high demand for mobile communication services is expected to occur such as in shopping malls, airports, etc.
A mobile terminal located in such a cellular communication network communicates with and/or via the network via an air interface between the terminal MS and the base station BS in a manner known as such in general and as for example set out in various GSM specifications, e.g. based on TDMA and/or CDMA etc.
As the terminal MS is mobile it may move at different speeds within the cellular network area. Also, when moving, it may cross one or more borders of the cells shown in FIG. 1. Upon crossing a cell border, the mobile terminal in most cases may require to be handed over to a new serving base station BS of the new cell to which it has moved. Such a handover is defined as a feature involving a change of physical channels, radio channels and/or terrestrial channels, involved in a call while maintaining a call. (A call being a logical association to/from the mobile terminal from/to a switch.) This change of channels might be required as caused by the movement of an active terminal (crossing a cell boundary) or caused by spectrum, user profile, capacity or network management issues.
Data exchange between the base station and the mobile station via the air interface (sometimes referred to as Um interface) according to GSM adopts, e.g. a time divisional multiple access scheme TDMA. According to TDMA, data are transmitted in units of bursts during consecutive time slots TS. Eight time slots according to GSM form one frame. One frame according to GSM has a duration of 4.615 ms. It is, however, to be noted that the present specification refers to GSM specific features only for explanatory purposes and other TDMA methods (for example adopting another number of time slots per frame, or another time duration per frame) may likewise be used in connection with the present invention. For example, the present invention as to be described later is easily applicable to the American IS-54 digital cellular system adopting a TDMA scheme with 6 time slots per frame and a frame duration of 40 ms, or even to the Japanese digital cellular system having a 3 channel TDMA multiple access scheme (full rate).
With regard to the GSM system again, individual frames are grouped in to multiframes. Dependent on the type of signaling transmitted in the multiframes, two types of multiframes can be distinguished:
1) for traffic channels carrying/transmitting (mainly) user data, 26 frames form a 26-multiframe (duration 120 ms), while
2) for signaling channels carrying/transmitting (only) control signaling information, 51 frames form a 51-multiframe (duration 235.38 ms).
Furthermore, 26*51 frames make up one superframe (duration 6.12 s), while 2048 times a superframe constitutes a hyperframe.
FIG. 3 illustrates an example of a 26-multiframe for a traffic channel. The 26 frames are numbered from #0 to #25. In the first 12 frames (#0 to #11) user data traffic is carried, frame #12 carries the SACCH (slow associated control channel, an inband control channel assigned to the traffic channel TCH or the slow dedicated control channel SDCCH). Frames #13 to 24 carry again user data traffic, and frame #25 is an idle frame which is not used for transmission.
Rather, the idle frame is required to be reserved for terminals for decoding SCH (synchronization channel) data transmitted in a 51-multiframe from the base station to the mobile terminal.
More precisely, in GSM and/or GSM/EDGE networks (EDGE=Enhanced Date rates for GSM Evolution, GSM=Global System for Mobile communications), as mentioned above, signaling information is carried in 51-multiframes. For example, in downlink direction (from BS to MS) in a combination of logical channels containing the SCH, the SCH is always transmitted in frames number #1, #11, #21, #31, and #41, respectively, i.e. five times per 51-multiframe. More precisely, the 51-multiframe is applied in the time slot 0 of the BCCH, or control channel, frequency.
In GSM/EDGE networks, on the SCH, cell identity is transmitted, and as mentioned above it takes place in 5 frames in each 51-frame control channel multiframe. As the networks are typically non-synchronized, a full idle frame must be reserved for terminals for SCH data decoding purposes. A full idle frame is necessary even in a synchronized network, because one's call can take place in the time slot which coincides with time slot 0 of the target cell. Cell identities must be established in order to attach signal level measurements to a particular neighbor cell. The cell identity is transmitted as the base station identity code BSIC. The BSIC is an identifier for the BS although the BSIC does not uniquely identify a single BS, since it has to be reused several times per PLMN network (public land mobile network). The BSIC serves for identification and distinction among neighbor cells, even when neighbor cells use the same BCCH (broadcast control channel) frequency. Since the BSIC is broadcast from the BS, the mobile terminal does not even need to establish a connection to the BS in order to retrieve the BSIC. The BSIC in turn consists of the network color code NCC identifying the PLMN and the base station color code BCC (3 bit) used to distinguish among eight different training sequence codes that one BS may use and to distinguish between eight neighboring base stations without a need for the mobile terminal to register on any other BS.
On full rate channels (FR), one frame in each 26-frame TCH multiframe is reserved for this purpose of SCH decoding, as seen from FIG. 3.
However, as the relative phases of TCH and control channel multiframes are random, in the worst case on a FR channel, one must attempt SCH decoding 11 times before it may be performed successful. The duration of this process is approximately 1.32 seconds. The reason therefore is that only after 286 frames (=11*26 multiframes) there occurs (for the first time) a coincidence and/or full overlap in time between a SCH frame in a 51-multiframe and an idle frame in a 26-multiframe. Thus, a delay in decoding of 286*4.615 ms=1319.89 ms≈1.32 s is caused.
Thus, as set out above, in GSM/EDGE cellular networks there is a delay in decoding the SCH data from a new neighbor cell. In the worst case it can be about 11 traffic channel (TCH) multiframes, or 1.32 seconds.
In preparation for a handover, however, one must decode SCH data from several neighbor cells and perform a number of signal level measurements on the neighbors. In cellular networks adopting e.g. a microcell network arrangement, fast moving mobiles may require frequent inter-cell handovers due to frequent cell border crossings.
Just as a numeric example, assume a microcell cellular network of microcells having a radius r=500 m. A mobile terminal starting to move from approximately the center of a cell would encounter a need for handover after (radially) traveling a distance of about r=500 m. Assuming further a speed of 100 km/h (=27.7 m/s), the mobile terminal would reach the microcell border after about 18 seconds. Assuming further that 6 neighbor base stations are to be monitored, 6*1.32 s=7.92 s were required for decoding/measuring the SCH of the neighbor BS which, being about half the time the mobile terminal needs for traveling, is quite too long for taking a decision concerning handover.
Such delays in neighbor cell SCH decoding and level measurement may thus result in incomplete data for inter-cell handover decision or even unsuccessfull handovers.
Previously, a common approach resided in locating fast moving cells in macrocells. This means that a fast moving mobile terminal was assigned to and handed over to base stations BS serving macrocells only (cells denoted with capital letter in FIG. 1).
This, however, is not a feasible solution in networks or areas, where only the microcell network layout exists (cells denoted with lowercase letter in FIG. 1).